Patent GB-711812-A (GB)
Title: Improvements in electrolytic determination of the concentration of a constituent in a solution
Inventor: ECKFELDT EDGAR LAWRENCE
IPC: G01N27/416
Filed: 1951-11-06
Abstract: 711,812. Measuring physical qualities electrically. LEEDS & NORTHRUP CO. Nov. 6, 1951, No. 25974/51. Class 37. Apparatus for determining the concentration of a constituent in a liquid, e:g. chlorine in water, comprises a chamber 14 (Fig. 1) containing the test liquid, through which a measured electrolyzing current is passed by means of electrodes 21, 22, the potential between a further pair of electrodes 29, 30 in the liquid being maintained at a value corresponding to an end point in the electrotitration process. In some embodiments of the invention a solution of 20 per cent ferric sulphate and 20 per cent sulphuric acid is added from a chamber 18, whereby the electrosis current converts the ferric ions to ferrous ions which react with the chlorine to produce chloride ions and ferric ions. Thus the ferric sulphate acts as a catalyst. For a given bulk of test liquid the coulombs, measured on a meter 25, required to reduce the potential between 29, 30 to the known end point is a measure of the concentration of the constituent. The potential between 29, 30 is determined by opposing it with a required proportion of the voltage across a potentiometer 31, and noting. the reading on a series galvanometer, which at the end point of the process reads zero. When applied to a liquid flowing at a constant rate, being fed through a constant-head chamber 12, the magnitude of the electrolyzing current is adjusted by a motor 37 whose energizing circuit is closed by a switch operated by the galvanometer 35, to vary a resistor 24 in the electrolyzing current circuit, the resulting current reading on meter 25 being indicative of the concentration. The reaction products at electrode 21 are isolated from the liquid in the chamber 14 by a porous diaphragm 27a. For determining the concentration of chlorine in water, electrode 21 may be silver and 22 platinum. The end point of the process may be determined by a pH measurement when electrodes 29, 3o may be known measurement and reference electrodes. Oxidizable or reducible impurities in the ferric sulphate reagent produce a constant error in the readings of meter 25 for which allowance must be made, or a suitably pure ferric sulphate may be used, made by a method described. In an embodiment wherein more accurate control of the electrolyzing current is obtained, the potential between the detecting electrodes 60, 61 (Fig. 4) is opposed by a fixed part of the EMF of a standard cell 6 9 , any unbalance producing a PD across a resistor  7 6 which is converted to A.C. by a vibrator 74 and fed into a three stage amplifier comprising valves 72, 83, 84. The input circuit also includes variable resistor 77 and condenser 78 which introduce an adjustable rate of change component into the amplifier input. The last stage 84 of the amplifier is followed by a phase inverter 86, and the anode voltages of 84, 86 are taken alternately to capacitors 87, 88 by synchronous vibrator 89. The polarity of the voltage across capacitors 87, 88 in series is dependent on the polarity of the voltage across resistor 76, and is fed directly to the electrodes 57, 58 as one source of electrolyzing current, the other source being obtained from B + through a potentiometer 96 whose slider 96a is adjusted in accordance with the magnitude and polarity of the amplifier output voltage across capacitors 87, 88. This voltage is developed across capacitor 106, connected between the grids of two negatively biased thyratrons 99, ioo, and, according to its polarity, causes 99 or 100 to fire, producing an operating pulse through relay winding 112a or 116a respectively, thereby stepping a toothed wheel 113 coupled to slider 96a clockwise or anticlockwise respectively, relay contacts 112b or 116b simultaneously discharging capacitor 106. This occurs whenever the voltage detected by electrodes 60, 61 is different from that set up by the slider of potentiometer 68. The position of the slider 96a is thus indicative of the concentration of the constituent of the liquid in chamber 50. In a modification of this embodiment, Fig. 5, (not shown) the amplifier output voltage is applied to the electrolyzing electrodes 57, 58 through a magnetic amplifier, the position of slider 96a also controlling the electrolizing current by varying the current through additional control windings of the magnetic amplifier. The method and apparatus described is said to be applicable to determine the concentration of e.g. potassium permanganate in a solution, and the catalytic reagents may be other than ferric salts, e.g. chromic salts, cupric salts, &c.